Numbers which are mutually equal to the sum of all their aliquot parts: as 220, 284. The aliquot parts of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110, the sum of which is 284. Again, the aliquot parts of 284 are 1, 2, 4, 71, 142, the sum of which is 220.